Wei Zhang, M.A.

Ian S. Abramson, Ph.D., Professor of Mathematics

October, 2001

To appear in the British Medical Journal, December, 2001

Phillips, Liu, Kwok, Jarvinen, Zhang (Department of Sociology)
and Abramson (Department of Mathematics) are from the University of
California at San Diego. Address reprint requests to Professor Phillips
at Sociology Department 0533, University of California at San Diego,
La Jolla, CA 92093-0533 (Tel: 858-534-0482; Fax: 858-534-4753; Email:
dphillip@weber.ucsd.edu).

What is already known on this topic:

Laboratory studies show cardiovascular changes following mild
psychological stress. However, these studies do not reveal whether, in the
world outside the laboratory, fatal heart attacks increase after psychological stress.
Previous nonlaboratory studies were unable to control for physical
and medical changes associated with most stressful occasions, and therefore were
not able to indicate whether psychological stress per se could trigger fatal heart
attacks.

What this study adds:

Unlike whites, Chinese and Japanese associate
"four" and the fourth with death. Unlike whites, Chinese/Japanese cardiac deaths
peak on the fourth day of the month. Because the fourth of the month is not objectively more hazardous than other
days of the month, our data may provide the best evidence to date that cardiac mortality increases
after psychological stress.

In The Hound of the Baskervilles, Charles Baskerville dies from
a heart attack induced by stress. Because our data suggest that heart attacks
increase on stressful occasions, our findings suggest that the "Baskerville effect"
exists in fact as well as in fiction.

Abstract

Objective: To determine whether cardiac mortality is abnormally high on days considered unlucky.

Design: The study group consisted of Chinese and Japanese (who consider
the number "four" unlucky) and white controls (who do not). Cardiac and
non-cardiac mortality on and around the fourth of each month was examined
for each group.

Setting: Entire United States.

Subjects: All U.S. Chinese and Japanese (n=209,908) and whites
(n=47,328,762) whose computerized death certificates were recorded
between 1/1/1973 to 12/31/1998.

Main outcome measures: Ratio of observed number of deaths on day 4 to
expected number of deaths on day 4; the expected number was estimated
from mortality on other days of the month.

Results: Chinese and Japanese cardiac mortality peaks on the fourth of the
month. The fourth-day peak is particularly large for deaths from chronic
heart disease (13% above expected; 95% confidence interval, 6%-21%) and
still larger for chronic heart deaths in California (27% above expected;
15%-39%). Within this group, inpatients display a particularly large,
47%, fourth-day peak (19%-81%). The fourth-day peak is not followed by
a compensatory drop in deaths below normal. White controls, matched on
age, sex, marital status, hospital status, location, and cause of death,
display no fourth-day peak in cardiac mortality.

Conclusions: Chinese and Japanese cardiac mortality is significantly
higher on the fourth than on any other day of the month. The results
are inconsistent with nine alternative explanations for the findings.
For example, the fourth-day peak does not seem to occur because of changes
in the patient's diet, alcohol intake, exercise, or medication regimes.
Our findings are consistent with the hypothesis that cardiac mortality
increases on psychologically stressful occasions.

Introduction

In The Hound of the Baskervilles, by Conan Doyle, [1] Charles
Baskerville has a fatal heart attack resulting from extreme
psychological stress. Conan Doyle was a physician as well as
an author; was his story based on medical intuition or literary
license? Are fatal heart attacks and stress linked in fact as
well as in fiction? Doyle's intuition is consistent with many
laboratory studies, [2-4] which show cardiovascular changes following
psychological stress. However, for ethical reasons, only non-fatal
stressors can be studied in the laboratory, and one may not be able
to generalize beyond these relatively mild stressors [5] to determine
whether, in the real world, fatal heart attacks increase immediately
after psychological stress.

One way to circumvent the ethical problems of the laboratory
experiment, and yet retain some of its rigor, is to use a natural
experiment [6], one that seeks psychological effects of a real-world
event occurring simultaneously to a study group (which is stressed
by the event) and to a control group (which is not). To separate
the effects of psychological factors from physical changes in
the environment, the event under study should not coincide with
destruction of the physical environment (as in disasters) or with
disruption of medical services. [7] In addition, the event should occur
several times, because it is easier to correct for accidental,
confounding factors when studying recurring events. We found no
previous studies meeting all these criteria. [3-5,7,8]

We approached the problem by identifying a cultural
phenomenon that has unpleasant associations for one group (Chinese and
Japanese) and neutral associations for another (whites). In Mandarin,
Cantonese, and Japanese, "death" and "four" are pronounced nearly
identically. [9,10] Extensive participant observation by three of our
authors (Liu, Kwok, and Zhang) indicates that the number "four" evokes
discomfort and apprehension in some Chinese/Japanese. Consequently, some
Chinese/Japanese hospitals do not list a fourth floor or rooms numbered
"four". [10,11] The Chinese omit the number "four" in designating military
aircraft-an omission said to result from the link between "four" and
death. [12] Some Japanese have reported avoiding travel on the fourth and
some Chinese patients have reported apprehension about this date (these
reports are based on participant observation). Aversion to the number
"four" is also evident from examination of Chinese/Japanese restaurants,
which avoid this number (as shown below).

This paper shows that Chinese/Japanese cardiac deaths peak on
the fourth; whites do not display this pattern.

Methods

We examined computerized daily U.S. mortality for Chinese/Japanese
(n=209,908) and whites (n=47,328,762) from 1973 (when daily mortality
was first recorded) to 1998 (the latest available data). [13,14]
Chinese, Japanese, and whites were identified from separate racial
codes on the death certificate.

For everyday life, American Asians use the Gregorian calendar.15
Therefore, we examined the Gregorian, rather than the ceremonial
lunar calendar. There is no correlation between day
of the month in one calendar and in the other; thus, the effects of day
of the month in the two calendars are not confounded.

Because U.S. mortality is slightly higher in the first week of each
month, [16] we used average daily mortality in the first week (days
1-3,5-7) to estimate the expected mortality level on day 4, given
the null hypothesis (H0.) Subsidiary analyses used days 1-3,5-28
to estimate expected mortality on day 4. The two approaches yield
nearly identical estimates of the number of deaths expected on day 4.

We measured the size of the fourth-day peak by R=X/(Y/6), where X is
the number of deaths on the fourth of the month, and Y is the total
number of deaths in the rest of the first week of the month. The
variance of R was determined from Table 3 in Gardner and Altman [17]
As in previous work [16,18,19,20] we followed recommendations in Vital
Statistics of the United States, [21] which justifies calculation of
confidence limits for complete population data.

Results

Statistical evidence for avoidance of the number "four"

New telephone subscribers in California have some choice of the last
four digits in their telephone numbers. In this four-digit section,
California restaurants listed as Chinese or Japanese [22] display
significantly (P=.00000003) fewer "fours" than expected (observed=366;
expected=4748/10=474.8; binomial test). California restaurants
listed as "American" [22] display no such pattern (P=.879; observed=219;
expected=2036/10=203.6; binomial test).

Asian and white cardiac mortality on and around the fourth

If the number "four" evokes superstitious stress in some Chinese/Japanese,
and if Doyle's medical intuitions were correct, Chinese/Japanese
cardiac mortality should peak on the fourth of each month. Doyle
suggests that Charles Baskerville was susceptible to a stress-induced
heart attack because he had a chronic heart condition. If so, chronic
heart disease should display a particularly large fourth-day peak. Sir
Charles' superstitious fear of an avenging, spectral hound was shared and
reinforced by his neighbors; similarly, Chinese/Japanese superstitious
fears are likely to be stronger where they are reinforced by large
Chinese/Japanese populations. Hence, the fourth-day peak is likely to be
stronger in California, which accounts for 42.6% of the Chinese/Japanese
deaths under study.

These expectations are supported by our data: On the fourth,
cardiac deaths are significantly more frequent than on any other
day of the month, and are 7.3% higher than the 6-day average
(Ratio=1.07, 95% confidence interval, 1.03-1.12). This percentage
increase (7.3%) is bigger for chronic heart deaths (13%; Ratio=1.13,
1.06-1.21) and still bigger (27%; Ratio=1.27, 1.15-1.39) for
chronic heart deaths in California. The fourth-day mortality peak
is henceforth termed the "Baskerville effect".

Whites do not display this effect, nor is it evident for
Chinese/Japanese who die from causes other than chronic heart disease
(for non-heart diseases: Ratio=1.02, .99-1.05; for heart diseases
other than chronic: Ratio=1.04, .98-1.09). Thus, Doyle's
medical intuition was remarkably precise: in our dataset, the fourth-day
peak occurs only in persons with pre-existing heart conditions.

Alternative Explanations for the Findings

Because U.S. mortality peaks in the first week of the month, we
used a six-day comparison period (1-3,5-7). Perhaps the Baskerville
effect is somehow an artifact of this comparison period. A regression
analysis, using days 1-3,5-28, generated a substitute for the 6-day
average. The 6-day and 27-day approaches always yielded very similar
Ratios and all findings remain statistically significant whichever
approach is used (see, e.g., Figure 1).

Perhaps the Baskerville effect occurs because superstitious
relatives attribute the decedent's death to the fourth when it
actually occurred on the third. Given this hypothesis, inpatients
should display a small or nonexistent fourth-day peak, because
the deathdates of inpatients are accurately recorded. However,
among California Chinese/Japanese dying from chronic heart disease,
inpatients display a larger fourth-day peak (Ratio=1.47, 1.19-1.81),
than do others (Ratio=1.16, .95-1.39).

On the fourth, patients may 1) change diets; 2) increase alcohol
consumption; 3) refuse medicines; or 4) overstrain themselves. These
behavioral changes are much less likely for inpatients (who are
closely controlled and monitored) than for others. The large
inpatient effect undermines all these explanations.

Perhaps the fourth-day peak in inpatient deaths occurs
because patients refuse to leave hospital on this unlucky day, thus
dying as inpatients rather than outpatients. This "discharge hypothesis"
implies a compensatory drop in outpatient deaths on the fourth, and no
such drop is found.

If the fourth-day peak occurred because deaths were merely
precipitated by a day or two, then this peak should be immediately
followed by a marked, compensatory drop in deaths. No such
drop is evident in mortality fluctuations throughout the month
(Figure 1A).

Perhaps the Baskerville effect appears because the study and
control groups differ demographically. To test this hypothesis,
we constructed a control group (Figure 1B) of whites matched to each
member of the study group (Figure 1A) on state and cause of death,
inpatient status, age, sex, and marital status. This matched control
group does not show the Baskerville effect.

It is unlikely that the fourth day is objectively more hazardous
than the six days surrounding it because white controls display no
increased mortality on the fourth.

Discussion

To assess the effects of psychological stress on cardiac mortality,
we identified a recurring, aversive occasion which 1) induces stress
in study groups but not in controls, and 2) is not objectively
hazardous.

This paper demonstrates that Chinese/Japanese cardiac deaths peak
on the fourth of the month. White controls display no such peak.
Presently, the only explanation consistent with the
findings is that psychological stress linked to the number "four"
elicits additional deaths among Chinese/Japanese.

There is no linguistic link in English between "thirteen" and
"death" and this may help to explain why white mortality shows
no peak on the thirteenth, despite the Chinese/Japanese peak on
the fourth.

The debate on whether there are fatal psychosomatic processes is
unresolved. The question deserves further investigation, provided
that such investigation uses rigorous methods, careful controls,
and large samples. The natural experiment presented here appears
to have met these criteria, and has provided a new technique for
examining fatal psychosomatic effects.

Our findings are consistent with the scientific literature and with
a famous, non-scientific story: The "Baskerville effect" exists
both in fact and in fiction, and suggests that Conan Doyle was not
only a great writer, but a remarkably intuitive physician as well.

We thank Peter H.T. Liu, A.M.D., Ph.D., Lien-Fen S. Chu, Rachel Phillips,
Chen Ruzhen, Daniel Smith, M.B.A., Elvira R. Strasser, Ph.D., Li Xianghui
for helpful comments.
Contributors: DPP coordinated and designed the study and wrote
most of the computer programs. GCL, KK, and WZ conducted informal
participant observation of Chinese and Japanese cultures and
performed most of the literature review. ISA calculated the
standard errors. All authors participated in the writing and
data analysis. DPP is the guarantor.
Funding: This study was supported in part by a grant from the Marian
E. Smith Foundation.
Competing Interests: None.

Figure 1. Number of California inpatient deaths from chronic heart
disease, by day of the month (1989-1998). The study period begins in
1989 because, prior to that date, inpatient status was seldom recorded
on California death certificates, [14] resulting in no pre-1989 recorded
Chinese/Japanese inpatient deaths from chronic heart disease. Deaths
from chronic heart disease are those coded 410-414 in the International
Classification of Diseases, 9th Revision. [23] The error bars represent
95% Poisson confidence intervals determined from Gardner and Altman,
Table 3. [17] The dotted line represents a regression line fitted to days
1-3,5-28. Panel A: Chinese and Japanese deaths. Panel B: Deaths for
white controls, matched to each Chinese and Japanese decedent on state
and cause of death, inpatient status, age, sex, and marital status.
For each Chinese/Japanese decedent we randomly selected twelve white
controls who had the same age, sex, and marital status. Twelve matches
were selected because it was possible to find at most twelve matches
for some of the Chinese/Japanese decedents. Chinese, Japanese and whites
were identified from race codes on the death certificate.